1. Field of the Invention
This invention relates generally to an electromagnetic drive motor assembly (EDMA). More particularly, the invention relates to a design of an EDMA that has dual coils positioned within a magnetic gap formed by a flux return assembly inside and an outside assembly of top and bottom plates sandwiching a permanent magnetic material. Enclosing the flux return assembly is a flux stabilization ring.
2. Description of the Related Art
Dual coil, dual gap electromagnetic motors or transducers have existed for many years, and recently they have been used more widely in the field of speaker design. Typically, an electromagnetic motor uses magnets to produce magnetic flux in an air gap. These magnets are typically permanent magnets, used in a magnetic circuit of ferromagnetic material to direct most of the flux produced by the permanent magnet through the components of the motor and into the air gap.
In one prior art transducer, a coil is placed in an air gap with its conductors wound cylindrically so as to be placed perpendicular to the magnet generating the magnetic flux in the air gap. The coil is normally connected to an audio amplifier of some type that produces a current in the coil that is a function of the electrical signal to be transformed by the loudspeaker into an audible, sub-audible or ultrasonic pressure variation. The coil is normally disposed to carry a current in a direction that is substantially perpendicular to the direction of the lines of magnetic flux produced by the magnet. The magnetic structure is often arranged to provide cylindrical symmetry with an annular air gap in which the magnet flux lines are directed radially with respect to the axis of cylindrical symmetry of the loudspeaker.
Other conventional electromagnetic loudspeakers employ a diaphragm that is vibrated by an electromechanical drive. The drive generally comprises a magnet and a voice coil with an electrical signal passed through the voice coil. The interaction between the current passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to oscillate in accordance with the electrical signal and, in turn, drives the diaphragm and produces sound.
One common problem associated with electromagnetic transducers is the generation and dissipation of heat. As current passes through the dual coils, the resistance of the components generates heat. The tolerance of the various internal components is limited by the thermal breakdown of those various components. If the internal components become too hot, the adhesives used to contact the components together, or the components themselves, will melt, resulting in a breakdown in the operation of the speaker. Since the resistance of the voice coil comprises a major portion of a driver""s impedance, most of the input power is converted into heat rather than sound. Thus, the power handling capacity of a motor is limited by its ability to tolerate heat.
Another problem associated with heat is temperature-induced resistance, commonly referred to as power compression. As the temperature of the motor voice coil increases, the resistance of conductors or wires used in the motor also increases. At higher temperatures, power input is converted mostly into additional heat rather than sound, thereby seriously limiting motor efficiency. When subjected to heat, the performance of some magnetic material can also be effected.
Another, problem is that the magnet in the transducer creates a static magnetic field, and this static field can be modulated by the changing magnetic field in the coil generated by the current in the voice coil. This phenomenon has been discussed by W. J. Cunningham, an article entitled xe2x80x9cNon-Linear Distortion in Dynamic Loudspeakers due to Magnetic Effects,xe2x80x9d J. Acoust. Soc. Am., Vol. 21, pp 207-207 (1949 May); and J. R. Gilliom, entitled xe2x80x9cDistortion in Dynamic Loudspeakers due to Modulation of the Permanent Field.xe2x80x9d presented at the 42nd Convention of the Audio Engineering Society, Los Angeles, Calif., 1972 May 2-5. Both of these references are hereby incorporated by reference into this application.
That is, as current is passed through the dual coils, the dual coils move within the static magnetic field. At the same time, the current passing through the dual coils also creates a magnetic field around the wire, as a result, the magnetic field around the wire moves within the static magnetic field of the magnet, thus modulating it. The amount of modulation is substantially related to the number of turns in the coil and current being applied, or the total Amp-tuns. Modulation of the static field can also be viewed as a xe2x80x9cglobalxe2x80x9d modulation effect that is asymmetrical and in turn generates second harmonic distortion. Depending on the saturation level of the steel surrounding the magnetic gap, the moving field of the voice coil also creates a xe2x80x9clocalxe2x80x9d modulation effect of the magnetic field within the steel surrounding the magnetic gaps so that symmetrical or third harmonic distortion is created in the output signal. Therefore, electromagnetic transducers must also be designed with the reduction of signal distortion in mind as well as heat dissipation.
Still another problem with the electromagnetic transducer is that air is often trapped underneath the dome or diaphragm so that as the dome moves up and down, the air is pressurized to resist the movement of the dome. Such resistance generates extraneous air noise, and is symmetrical in nature so that third harmonic distortion is generated.
Yet another problem is modulation within the magnetic gap. This is largely due to weak magnetic flux running through the steel that creates the magnetic gap and this causes third harmonic distortion due to a symmetrical phenomenon. And as discussed above this creates a xe2x80x9clocalxe2x80x9d modulation effect.
Accordingly, there still is a need for an electromagnetic motor that maximizes output power; dissipates heat well; minimize distortion; minimize modulation; and reduce air noise.
The present invention satisfies the above needs by providing an electromagnetic drive motor assembly (EDMA) that has an inner flux return assembly formed from an upper pole piece and a lower pole piece oppositely disposed relative to each other along the center of the EDMA. An annularly positioned conductive ring (or flux stabilization ring) encircles the inner flux return assembly, concentrically in contact with the outside of the inner flux return assembly. Along the exterior side of the flux return pieces, is a magnet that is between a top plate and a bottom plate. Each plate contacts with the magnet to form a magnetically conductive system. A magnetic gap is formed in between the inner flux return assembly and the top and bottom plates. Disposed within the magnetic gap is a dual coil wound around a cylinder. The dual coil includes a first coil portion and a second coil portion.
The top and bottom plates and the inner flux return assembly may be made of ferromagnetic material, such as steel, and may be designed to operate within the magnetic saturation region near the magnetic gap but below the magnetic saturation region throughout the other areas; but at the same time, they are optimized to minimize weight. The magnetic saturation near the magnetic gap reduces flux modulation in the ferromagnetic material directly adjacent to the coil thus minimizing distortion. On the other hand, there is a trade off between low distortion and higher output energy when comparing designs of the same weight of permanent magnetic material. Put differently, to design a xe2x80x9ccost efficientxe2x80x9d magnetic structure, the structure may be designed to use minimal steel as possible before the steel saturates to a point where there is loss of flux level in the magnetic gap. With such design, the inner flux return assembly, for example, may be designed with a hole along its center so that air within the cylinder is allowed to move in and out freely and not resist the movement of the woofer. This also reduces the air noise in the transducer generated by the resistance.
The flux stabilization ring, or conductive ring, performs multiple functions: (1) it concentrically holds the upper and lower pole pieces together, this reduces manufacturing cost and time; (2) draws heat away from the magnetic gap due to its proximity to the voice coils; and (3) substantially stabilizes the modulation effect caused by the magnetic fields around the voice coil moving within the static magnetic field from the magnet.
To efficiently transfer heat away from the magnetic gap, a thermal radiating conductive ring may be provided with an inner ring juxtaposed to the magnetic gap, an outer ring near the atmosphere, and a bridge between the inner and outer rings and running through the magnets. This way, the heat within the magnetic gap area can be more directly transferred from the inner ring and to the outer ring.
The top and bottom plates each may have an extended plate tip, or thin area of steel, to force magnetic saturation of the material adjacent to the voice coils. Again, the magnetic saturation of the plate and/or plate tips reduces the flux modulation in the steel directly adjacent to the coil thus minimizing distortion.
The above described and many other features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description when considered in conjunction with the accompanying drawings.